Explanation
Expansion rates are low everywhere, which means the visible portion has to carry the point on its own. The most common mistake is opening with hashtags, which spends the only guaranteed line on text nobody reads for meaning.
The hidden portion still has a job. TikTok and YouTube both index caption text well past the truncation point for search, so length is a discovery decision rather than a reader-facing one. Write the first line for a person and the rest for a query.
The common mistake
Confusing the caption with on-screen subtitles. They are different things with different jobs, and a caption that duplicates the on-screen text wastes one of the two slots.